Hi. I have a 1988 Snyder General furnace Model GDA060A012AIN with a White-Rogers Digital Comfort-Set II thermostat. My furnace blower is constantly running, even with the thermostat on/auto fan setting set to auto and the thermostat heat/cool/off option turned to off, it still runs even with the thermostat removed from its wall socket. The only way the blower is off is when power to the furnace is off or the blower door is open (the cover is removed).

My thought is that either the thermostat is sending a signal to the blower to run even when the blower is set to the off position (a bad thermostat), or the blower relay switch is bad and runs the blower even when no signal is present (bad blower relay). But based on the problem still happening when I remove the thermostat unit from the wall base I'm guessing that it is the blower relay.

What is the easiest way for me to figure out what is actually wrong? Besides calling a service technician.

does the fan change speed when you turn the fan switch from auto to on ? if so, with the fan switch in the auto position, give the furnace a hard bump. does the fan go off ?

if the fan does not change speed when changing the thermostat fan switch, the relay is the problem. if not repaired, the motor will short when you turn on the heat. if the fan goes off with the bump, the problem is a thermal fan switch controlling low speed fan that has closed due to excess ambient heat and it is hanging in the closed position. jarring it may open the switch and turn it off.

get this furnace tested thoroughly for cracks prior to use. if the bump works, replace that thermal control before use. consider replacement of the furnace. it will eventually present a safety hazard. you got your goodie out of it. stay safe and upgrade.